On an earlier post, DW asked about retroactive effects of the rulemaking.

A relevant excerpt:

So the FDA thinks this is already the law -- no surprise given the amicus briefs mentioned. So to some extent, this is a lot of hullabaloo (gosh, I like getting to use that word!) over something that's not really that big a deal. But it's a nice explicit and non-case-specific articulation of their view. Indeed, Dan Troy, former counsel at FDA (now at Sidley Austin Brown & Wood) made that position fairly clear, as I recall, at a DRI Drug & Medical Device conference I attended several years ago, and my sense is that the position has stayed consistent throughout the current administration.

So I think the answer is that if the FDA is right, it's not really even a question of retroactivity -- it's been that way all along, just that nobody really knew it. The fact that the position has generally been unsuccessful in the past may suggest that it won't be in the future, but who knows?